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Roughly 7% of New Zealand’s total landmass is covered by exotic plantation forests.

The forestry industry contributes an annual income of around $5 billion to our economy, makes up 3% of GDP, and provides jobs for close to 20,000 people.

Forestry is undeniably important to New Zealand – socially, economically, and even environmentally. As our government pushes towards an ambitious ‘carbon zero by 2050’ goal, the role of planted forests in removing CO2 from the atmosphere through photosynthesis (a process termed ‘carbon sequestration’) will become vital.

Source: USDA, Northern Research Station.

However, forestry also has its downsides, and one of them, the increasing occurrence of debris flows, needs to be addressed.

In the following Q+A we’ll take you through the ins and outs of debris flows, how they occur, what their effects are and how they might be managed more effectively in the future.

What are debris flows?

They tend to contain a 60-80% concentration of sedimentary material (matter that settles to the bottom of a water body, e.g. sand, silt, pebbles, clay) and may or may not also hold woody material from forestry.

Debris flows usually move in surges, growing gradually in mass through entrainment (surface sediment picked up by a fluid flow and incorporated into a larger body).

Debris avalanches or shallow landslides can transform into debris flows once they become confined to a channel.

How do they occur?

The primary cause of a debris flow, as described by the NZFOA, is the “generation of sediment by mass movement into a channel.”

This generally occurs when a land area characterised by high, steep slopes and plentiful sedimentary ground matter is hit by heavy rainfall, causing large volumes of debris to erode vertically into a channel. Once the flow begins, it is generally only halted by a change in gradient or a reduction in flow depth, e.g. - on a fan or where a steep stream exits onto a flood plain.

Planted forests which sit in the post-harvest, pre-canopy closure phase (which usually lasts 5-6 years) typically contain high volumes of sediment, putting them in what is known as the ‘window of vulnerability’ for debris flows.

During this time, the removal of the forest canopy leaves the soil exposed to rainfall, making it increasingly vulnerable to surface erosion and water infiltration. Additionally, as the trees have been felled, their remaining roots begin to rot, reducing their soil reinforcement capacity.

When a period of high rainfall hits an area of forest in the ‘window of vulnerability’, debris flows tend to occur.

What are the impacts of debris flows?

Debris flows from forestry can have devastating effects on nearby towns and infrastructure, a fact highlighted by two recent examples in New Zealand.

In February of this year, in Marahau, a period of extremely high rainfall (brought on by Cyclone Gita) caused a huge flow of sediment and forest debris to descend from the hills, smashing property fences, damaging buildings and exposing residents to serious injury.

Two months later, an even larger debris flow damaged a total of 61 bridges and culverts in the East Coast district. This, again, was a product of extremely high rainfall and vulnerable forestry land.

In the Mangakahia Valley, Northland, poorly planned planting in the ‘80s has created consistent issues with debris flow. In 2003, resident Bruce Alexander carried out a survey for Carter Holt of the damage caused to local waterways from planting on soft soil and steep slopes.

His photographs show pines planted right down to the riverbanks, toppling into the water, taking big chunks of the bank with them, turning the stream brown with silt and creating dams as waste timber built up behind them.

Overseas, debris flows have washed away entire villages, destroyed agricultural lands, resulted in fatalities and had environmental impacts like loss of habitat, increased siltation of waterways and changes in topography.

How can debris flows be minimised?

As the NZFOA states, debris flows are a natural process, and therefore they cannot and should not be prevented altogether. However, their increasing frequency – a product of plantation forestry – can be curbed, provided careful planning is put in place.

NZ Forestry identifies the following six measures as integral for reducing both the occurrence of debris flows and the environmental + economic damages they cause:

1) Hazard avoidance. Identify areas where debris flows are likely to occur and take note of infrastructure (roads, bridges, buildings) and people that may resultantly be affected.

2) Defence/channelling structures. In areas where risk to infrastructure and life is high, introduce structures (slash racks, driven railway irons) that may mitigate risk by trapping or channelling debris into storage areas for management and disposal.

3) Narrowing the window of opportunity. In order to reduce the 5-6 period in which forestry soils become vulnerable to water infiltration and erosion, replant at a higher stocking density. Additionally, look into planting species that maintain live roots after harvest.

4) Riparian management. Riparian planting on lower slopes has been known to trap and impede debris flows. It is important however, that careful logistical planning is undertaken to prevent riparian plants being entrained into the flow.

5) Longer rotation crops or retirement into permanent forest cover. Identify areas most vulnerable to landslides and debris flows and consider a change to rotation length or conversion to permanent forest cover.

6) Improved risk models. Current data on risks and mitigation options for forestry management is widely considered inadequate. Collection of standardised data on size, frequency and impact of debris flows as a part of routine forest monitoring operations will enable the improvement of risk models, and the more effective management and mitigation of the impact of debris flows.

The Labour government’s emissions-curbing plan to plant 1 billion trees over the next ten years will require careful planning to ensure the risk of debris flows, along with other afforestation-related hazards, are minimised.

The Eastland Wood Council’s 2018 publication “Our changing landscape” highlights the vulnerability of the Gisborne region to debris flows and emphasises the need to reduce their occurrence when further planting occurs there over the next decade:

“The Gisborne East Coast is part of a plate collision crush zone. It is geologically fragile, with our steep hillsides formed as a consequence of natural erosion and land uplift, something that has been going on for thousands of years and it is not stopping. Weak underlying geology, with recent thin soils on top, steep slopes, rapid down-cutting by rivers (due to the rate of uplift) and periodic high rainfall and high intensity storm events mean erosion rates are naturally high. The main form of erosion is mass movement — and under forests this will mean debris flow.”

Photos taken in the Tapuaeroa Valley show the effects of erosion from forestry. Credit - Eastland Wood Council.

It mentions the measures previously stated for risk mitigation, and additionally notes species change:

“Under the Government’s 1 Billion Trees programme there is an opportunity to look at other species, particularly for red-zoned land, and to understand the benefits of them. Many forest owners are also investigating tree species along riparian margins.”

Strategies like these will continue to be studied and trialed by the industry as it works with researchers, regulatory bodies, communities, and Government agencies to develop and improve best practice and ensure that the future planting of forests, both exotic and native, is something that helps – not hinders - people and the environment